For all his accomplishments and pedigree, Tuukka Rask has never been a full-time, No. 1 NHL goalie. The short-term contract Rask agreed to Thursday reflects both the optimism and caution with which the Bruins view the 25-year-old.

Rask agreed in principle to a one-year, $3.5 million extension, according to two sources familiar with the negotiations. He will officially sign the contract Sunday. On the same day, Chris Kelly will sign his four-year, $12 million extension.

Rask will be the team’s go-to netminder in 2012-13 because of Tim Thomas’s intention not to play next season.

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The Bruins originally acquired Rask, the 21st overall pick of the 2005 draft, from Toronto for Andrew Raycroft on June 24, 2006. Rask was coming off a two-year, $2.5 million contract.

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In 2012-13, Rask will be paid more than Detroit’s Jimmy Howard ($2.25 million) and Phoenix’s Mike Smith ($2 million), two rock-solid
goalies, because of how the Bruins expect him to perform. But Rask did not receive the term recently landed by Jonathan Quick (10 years, $58 million) of the Kings and Ondrej Pavelec (five years, $19.5 million) of the Jets.

Quick, Pavelec, and Montreal’s Carey Price (the latter is due for a long-term extension) are all young, established No. 1 goalies. Rask has yet to earn his long-term keep.

Rask will be a restricted free agent at the end of his new deal. Based on the one-year extension, both sides are treating 2012-13 as the season in which Rask can establish himself for a richer, long-term deal. If Rask delivers the goods next season, he will be in line for a raise.

The Finnish netminder projects to be an ace next season if he remains healthy and his defense is stout. Last season, Rask went 11-8-3 with a 2.05 goals-against average and a .929 save percentage. Rask’s season ended March 3 against the Islanders at TD Garden when he suffered a lower abdomen/groin strain while attempting a save. He did not require surgery and returned to back up Thomas by Game 6 of the first-round playoff series against Washington.

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Rask’s sharpest season was in 2009-10. With Thomas off his form, primarily because of a hip injury that would require offseason surgery, Rask grabbed the No. 1 job. In 45 games, he went 22-12-5 with a 1.97 GAA, .931 save percentage, and five shutouts. In the playoffs, Rask went 7-6 with a 2.61 GAA and .912 save percentage. Rask was sharp in the first round of the playoffs against Buffalo, but faded in the second round against Philadelphia.

By signing Rask, the sides avoid arbitration and the threat of an offer sheet. Between Sunday, when Rask would have reached RFA status, and Thursday, any team could have signed Rask to an offer sheet. The Bruins would have matched, but might have been forced to exceed their salary structure by doing so.

Thursday, Rask could have filed for arbitration. The Bruins would have had team-elected arbitration rights on July 6.

Rask will be backed up by Anton Khudobin. Both have recent injury histories. Two summers ago, Rask required arthroscopic surgery on his left knee. Khudobin would have been recalled to back up Thomas after Rask suffered his abdomen/groin injury, but was on the shelf because of an injured wrist.

Once Rask and Kelly sign their deals, the Bruins will have committed approximately $67 million in salaries toward 2012-13. That includes the annual cap hits of Thomas and Marc Savard.

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On Thursday, the 2012-13 cap was set at $70.2 million. Teams can exceed the cap by 10 percent during the offseason. However, the cap could decline upon the introduction of the new collective bargaining agreement.

Fluto Shinzawa can be reached at fshinzawa@globe.com. Follow him on Twitter @GlobeFluto.